1. Field of the Invention
The invention relates to a vitreous silica envelope tube for the preparation of a light guide fiber billet.
2. Discussion of Prior Art
Light guide fibers are produced by the drawing of especially prepared blanks or rods. These rods are built up of concentric layers having different properties required for the particular purpose. The optical transmission properties of light guide fibers are controlled by the internal layers, such as the core and the sheath. The latter are then surrounded by an envelope which improves the mechanical strength, especially the tensile strength, of the light guide fibers.
For the production of such rods for the drawing of light guide fibers, a rod having the desired properties can be produced either by the internal coating of an envelope tube, or by encasing a finished rod consisting of core and sheath in an envelope tube and welding the two together. Tubes of fused rock crystal or quartz sand or tubes of synthetic quartz glass (vitreous fused silica) are used in making the envelope tubes.
These known quartz glass envelope tubes made of fused rock crystal or quartz sand are prepared from materials of high purity. In spite of this, these envelope tubes still contain mineral impurities and bubbles which limit the tensile strength as well as the maximum producible fiber length, a disadvantage which is felt particularly in the production of light guide fibers for undersea cables.
It is known to produce envelope tubes also from synthetic quartz glass. These tubes make possible a greater fiber length and higher tensile strength in the light guide fibers. The production of these envelopes is very expensive, however, on account of the amount of labor involved and also on account of the high cost of the material.